Il6 AN INTRODUCTION TO THE STUDY OF FOSSILS 



These extinct Hydrozoa are known only from their colonial 

 phase, the presence of the jelly-fish phase not yet having been 

 demonstrated. Some forms, at least, as Diplograptus, devel- 

 oped direct, without the alternate generation ; here generative 

 cysts formed on the colony, in which developed the siculae 

 (embryonic graptolites) ; the rupture of these cysts scattered 

 the siculae ; each surviving sicula grew^ into a new colony. 



Habitat. — Graptolites, with the exception of some of those 

 belonging to the sub-order Dendroidea, lived suspended. That 

 they did not live fastened to the sea-bottom is shown by the 

 fact that their remains are always confined to one bedding plane, 

 never passing vertically from one bed to another. They are 

 always spread fiat as though they had fallen with the entombing 

 sediment. While they are found in all kinds of sediment, in- 

 cluding limestones, they are most numerous in very fine-grained, 

 carbonaceous shales. The carbon of the shales was probably de- 

 rived from floating marine plants, not from the abundant grapto- 

 lite remains, for seldom, if ever, do decomposed rhabdosomes 

 pass into the surrounding shales. Since few land plants were prob- 

 ably in existence during Ordovician times, — the climax of grap- 

 tolite development, — it is likely that the carbon is due to the par- 

 tial decay of marine plants. Many fragments of seaweed occur 

 in such typical graptoHte beds as the Utica shale of the Ordo- 

 vician of the Appalachian region. Suspended from these 

 plants were representatives of the Axonolipa division of the 

 Graptoloidea, while floating among them were Axonophora 

 colonies supported by their pneumatocysts. The latter prob- 

 ably sank during periods of roughened seas, as some of them 

 had developed ridges w^hich might have subserved such a 

 directive function. A roughening of the water surface would 

 cause a rain of the broken, fragile colonies upon the sediment 

 beneath. This method of growth accounts likewise for the 

 wide distribution of so very many of the species. That even 

 the Graptoloidea sub-order was derived from forms which grew 

 upright upon the sea-bottom as did many of the Dendroidea 



